The IGF-1 receptor signaling system plays an important role in tumor proliferation and survival and is implicated in inhibition of tumor apoptosis. In addition and independent of its mitogenic properties, IGF-1R activation can protect against or at least retard programmed cell death in vitro and in vivo (Harrington et al., EMBO J. 13 (1994) 3286–3295; Sell et al., Cancer Res. 55 (1995) 303–305; Singleton et al., Cancer Res. 56 (1996) 4522–4529). A decrease in the level of IGF-1R below wild type levels was also shown to cause massive apoptosis of tumor cells in vivo (Resnicoff et al., Cancer Res. 55 (1995) 2463–2469; Resnicoff et al., Cancer Res. 55 (1995) 3739–3741). Overexpression of either ligand (IGF) and/or the receptor is a feature of various tumor cell lines and can lead to tumor formation in animal models. Overexpression of human IGF-1R resulted in ligand-dependent anchorage-independent growth of NIH 3T3 or Rat-1 fibroblasts and inoculation of these cells caused a rapid tumor formation in nude mice (Kaleko et al., Mol. Cell. Biol. 10 (1990) 464–473; Prager et al., Proc. Natl. Acad. Sci. USA 91 (1994) 2181–2185). Transgenic mice overexpressing IGF-II specifically in the mammary gland develop mammary adenocarcinoma (Bates et al., Br. J. Cancer 72 (1995) 1189–1193) and transgenic mice overexpressing IGF-II under the control of a more general promoter develop an elevated number and wide spectrum of tumor types (Rogler et al., J. Biol. Chem. 269 (1994) 13779–13784). One example among many for human tumors overexpressing IGF-I or IGF-II at very high frequency (>80%) are Small Cell Lung Carcinomas (Quinn et al., J. Biol. Chem. 271 (1996) 11477–11483). Signaling by the IGF system seems to be also required for the transforming activity of certain oncogenes. Fetal fibroblasts with a disruption of the IGF-1R gene cannot be transformed by the SV40 T antigen, activated Ha-ras, or a combination of both (Sell et al., Proc. Natl. Acad. Sci. USA 90 (1993) 11217–11221; Sell et al., Mol. Cell. Biol. 14 (1994) 3604–3612), and the E5 protein of the bovine papilloma virus is also no longer transforming (Morrione et al., J. Virol. 69 (1995) 5300–5303). Interference with the IGF/IGF-1R system was also shown to reverse the transformed phenotype and to inhibit tumor growth (Trojan et al., Science 259 (1993) 94–97; Kalebic et al., Cancer Res. 54 (1994) 5531–5534; Prager et al., Proc. Natl. Acad, Sci. USA 91 (1994) 2181–2185; Resnicoff et al., Cancer Res. 54 (1994) 2218–2222; Resnicoff et al., Cancer Res. 54 (1994) 4848–4850; Resnicoff et al., Cancer Res. 55 (1995) 2463–2469. For example, mice injected with rat prostate adenocarcinoma cells (PA-III) transfected with IGF-1R antisense cDNA (729 bp) develop tumors 90% smaller than controls or remained tumor-free after 60 days of observation (Burfeind et al., Proc. Natl. Acad. Sci. USA 93 (1996) 7263–7268). IGF-1R mediated protection against apoptosis is independent of de-novo gene expression and protein synthesis. Thus, IGF-1 likely exerts its anti-apoptotic function via the activation of preformed cytosolic mediators.
Some signaling substrates which bind to the IGF-1R (e.g. IRS-1, SHC, p85 PI3 kinase etc., for details see below) have been described. However, none of these transducers is unique to the IGF-1R and thus could be exclusively responsible for the unique biological features of the IGF-1R compared to other receptor tyrosine kinase including the insulin receptor. This indicates that specific targets of the IGF-1R (or at least the IGF-receptor subfamily) might exist which trigger survival and counteract apoptosis and thus are prime pharmaceutical targets for anti-cancer therapy.
By using the yeast two-hybrid system it was shown that p85, the regulatory domain of phosphatidyl inositol 3 kinase (PI3K), interacts with the IGF-1R (Lamothe, B., et al., FEBS Lett. 373 (1995) 51–55; Tartare-Decker, S., et al., Endocrinology 137 (1996) 1019–1024). However binding of p85 to many other receptor tyrosine kinases of virtually all families is also seen. Another binding partner of the IGF-1R defined by two-hybrid screening is SHC which binds also to other tyrosine kinases such as trk, met, EGF-R and the insulin receptor (Tartare-Deckert, S., et al., J. Biol. Chem. 270 (1995) 23456–23460). The insulin receptor substrate 1 (IRS-1) and insulin receptor substrate 2 (IRS-2) were also found to both interact with the IGF-1R as well as the insulin receptor (Tartare-Deckert, S., et al., J. Biol. Chem. 270 (1995) 23456–23460; He, W., et al., J. Biol. Chem. 271 (1996) 11641–11645; Dey, R. B., et al., Mol. Endocrinol. 10 (1996) 631–641). Grb 10 which interacts with the IGF-1R also shares many tyrosine kinases as binding partners, e.g. met, insulin receptor, kit and abl (Dey, R. B., et al., Mol. Endocrinol. 10 (1996) 631–641; Morrione, A., et al., Cancer Res. 56 (1996) 3165–3167). The phosphatase PTP1D (syp) shows also a very promiscuous binding capacity, i.e. binds to IGF-1R, insulin receptor, met and others (Rocchi, S., et al., Endocrinology 137 (1996) 4944–4952). More recently, mSH2-B and vav were described as binders of the IGF-1R, but interaction is also seen with other tyrosine kinases, e.g. mSH2-B also bind to ret and the insulin receptor (Wang, J., and Riedel, H., J. Biol. Chem. 273 (1998) 3136–3139). Taken together, the so far described IGF-1R binding proteins represent relatively unspecific targets for therapeutic approaches, or are in the case of the insulin receptor substrates (IRS-1, IRS-2) indispensable for insulin-driven activities.